The present invention relates to wrenches, and relates more particularly to such a wrench which can magnify the index number of torsion for an accurate calibration.
For turning a screw or a nut, a wrench or the like may be used. In certain cases, the tightness of the screw or nut must be accurately controlled. If an excessive torsional force is applied to the screw or nut, the thread tend to be damaged. If the thread of the screw or nut is damaged, the required air tight condition cannot be achieved. However, it is difficult to accurately control the tightness of the screw or nut when it is turned by a regular wrench. There is known a wrench which can be adjusted to calibrate the desired torsional force. This structure of wrench comprises an operation arm having an operation head at one end for turning a workpiece, a calibration handle connected to the operation arm at one end remote from the operation arm and turned relative to the operation arm to set the desired index number of torsion. When the reaction force from the screw or nut surpasses the set torsional force, the wrench will run idle. This structure of wrench is functional. However, because the indexes on the operation arm adjacent to the calibration handle and the graduations on the calibration handle are made of small scale, they cannot be easily distinguished. Therefore, a reading error tends to occur during the calibration.